The objective of this proposal is to provide a deeper molecular understanding of certain key processes in mammalian development and embryology. A clearer understanding of normal development will, of course, impact greatly on our ability to comprehend and deal with the abnormal events resulting in birth defects and proliferative diseases. The main premise of this proposal is that the study of mammalian development can benefit greatly from a combined approach that employs the tools of embryology, genetics and recombinant DNA methodology. In particular, analysis of genes of central importance in development may be quite enlightening. This laboratory has been extremely fortunate in its recent identification of a transgenic line of mice in which such a gene, of key significance in development, has been disrupted. Mice with two copies of the interrupted gene are born with hindlimbs that truncate abruptly at the femur, with no distal structures present. Forelimbs are missing anterior digits and while one bone of the forearm, the ulna, is always normal, the bone beside it, the radius, is often dramatically reduced or absent. The brains are also malformed, with missing olfactory lobes and necrotic lesions in the anterior cerebrum. Embryological analysis is beginning to shed light on the nature of the defect at the cellular level. An abnormal wave of mesenchymal cell death is observed during early hindlimb development. Furthermore because this gene, named legless, carries the foreign transgene "tag" within it, it can readily be cloned and studied to better understand its role in development. The timing and positional specificity of gene expression can be determined. The encoded protein can be made, analyzed and localized within the developing embryo. Moreover, the gene can be systematically altered in vitro and then functionally dissected by re-introducing it into mutant mice and observing the partial or complete rescue of normal development. The proposed embryological, genetic and molecular study is only possible because of the presence of the transgenic line, which allows continual production of abnormal mice for embryological study, as well as molecular isolation and characterization of the altered gene. Each aspect of analysis complements the others, providing a more complete understanding of the developmental process.